At present, in a 100-G transmission system being put into practical use in an optical fiber communication system, a quaternary phase modulation (Quadrature Phase-Shift Keying: QPSK modulation) system which is one type of multilevel modulation technology and a polarization multiplexing system which is one type of multiplexing technology are used to realize a high speed transmission rate of 128 Gbps per channel. The multilevel modulation technology is a technology in which not only the amplitude of an optical signal but also phase information is utilized and thus a large number of pieces of information are transmitted with one symbol. In the QPSK modulation system, symbols are allocated to four levels, that is, four types of phase states, and thus information of 2 bits is transmitted with one symbol. The polarization multiplexing system is a technology in which polarization is utilized to multiplex and simultaneously transmit signals in 2 series, and it is possible to obtain twice as high a transmission rate as in the transmission system using normal single polarization. Hence, in a 100 G transmission system in the polarization multiplexing QPSK modulation system, it is possible to transmit information of a total of 4 bits per symbol and reduce a symbol rate to 32 Gbaud which is one-fourth of the bit rate. Whether the symbol rate is high or low affects not only the magnitude of transmission degradation such as polarization mode dispersion but also a requirement for characteristic of a modulator and a demodulator, and thus becomes an indicator that significantly affects the realization of the system.
In research and development for future transmission system exceeding 100 G, in order for the transmission rate to be increased while the symbol rate is kept low, a QAM modulation system in which the multilevel number is further increased and a subcarrier multiplexing system using a plurality of transmission carriers are examined. For example, in a 16QAM modulation system, symbols are allocated to 16-ary, that is, 16 types of phase amplitude states on a signal space diagram, and thus it is possible to transmit information of four bits with one symbol. In a 2-subcarrier multiplexing system, since information can be transmitted independently in two carriers, it is possible to transmit twice as high a rate as in a normal 1 carrier transmission system. The subcarrier multiplexing system is also said to be Orthogonal Frequency-Division Multiplexing (OFDM) when carriers are arranged at the minimum spacing which maintains an orthogonal relationship between the carriers, that is, the same carrier intervals as the symbol rate.
Before the description of modulators that generate optical signals of these systems, a QPSK modulator that forms a basic configuration of these modulators will first be described with reference to FIGS. 1A to 3D. FIG. 1A shows an example of the configuration of the QPSK modulator; FIG. 1B shows, as the outline of the operation of the QPSK modulator of FIG. 1A, the intensity waveform and the signal constellation of the optical signal at each of points A to G in the QPSK modulator. A QPSK modulator 1 has a configuration called a nest MZI modulator in which a MZI modulator (child MZI modulator) is nested into each arm waveguide part of a Mach-Zehnder Interferometer (MZI), which will be called a parent MZI.
FIGS. 2A to 2D show the operation of the child MZI, that is, single MZI modulator in detail. Although in FIGS. 1A and 2A, the modulator is assumed to be an LN modulator using a Z-cut substrate of lithium niobate (LiNbO3; LN), the same operation is basically performed also when an X-cut substrate is used. As shown in FIG. 2A, when the Z-cut substrate is used, a drive electrical data signal Vdrv2 is divided into two outputs by a differential output drive circuit 3, differential input signal (+Vdrv/2 for a lower arm, −Vdrv/2 for the lower arm) is fed to an upper arm optical phase shifter for modulation 4 and a lower arm optical phase shifter for modulation 5 of the modulator and so-called push-pull driving is performed. When the X-cut substrate is used, since a drive electrode is normally arranged between the modulator arms, and opposite electric fields are applied to the upper and lower arms, the push-pull driving is automatically performed. Continuous (Continuous Wave; CW) light 6 launched to the single MZI modulator is branched into two by a 3 dB optical coupler 7a, and then the branched lights are phase-modulated in the respective upper and lower arm optical phase shifters for modulation 4 and 5 and are combined again at a 3 dB optical coupler 7f. The aspect of the phase modulation in signal space diagram at this time is shown in FIG. 2B. An arrow shown in FIG. 2B indicates an electric field vector of an output signal light 8. Since the light through the upper arm is phase-modulated in a positive direction, its electric field vector tracks a counterclockwise trajectory (cross→white circle→black circle), whereas the light through the lower arm is phase-modulated in a negative direction, its electric field vector tracks a clockwise trajectory. Since the resultant vector of both electric field vectors is the electric field vector of the output signal light, the output signal light tracks a straight trajectory on the real axis. Here, as shown in FIG. 2C, when the single MZI modulator is driven by a data electrical signal so that the phase difference between the arm waveguides is changed by 2π, the output light is phase-modulated at phase 0 and phase π, and is modulated into two phase values in which the intensity of the signal light is a constant value at signal timing. As described above, the single MZI modulator operates as an optical phase modulator. In the optical phase modulation by the single MZI modulator described above, as compared with the optical phase modulation by an optical phase modulator including only a simple optical phase shifter, even if the drive amplitude of the electrical data signal is slightly varied, an optical signal output advantageously little varies by the nonlinear behavior of the MZI as is found from FIG. 2C, and, since a property in which the a modulation spectrum is narrowed is provided, it is suitably used as a modulator that generates a phase modulation (Phase-Shift Keying: PSK) signal. FIG. 2D is a diagram schematically showing a single MZI modulator 13.
In the nest MZI modulator, as shown in FIGS. 1A and 1B, the CW lights branched by a 3 dB coupler 7a are respectively binary phase-modulated by the child MZI modulators (an MZI modulator for Ich 9 and an MZI modulator for Qch 10) (see D and E in FIG. 1B) and take a 90° phase shift with a π/2 optical phase shifter 11 (see F in FIG. 1B), and those modulation signals are combined by a 3 dB coupler 7f to give a quaternary phase-modulated QPSK signal light as shown in G of FIG. 1B. Note that, the π/2 optical phase shifter is actually realized by the adjustment with a subsequent phase adjuster (variable optical phase shifter) 12, and the π/2 optical phase shifter is often omitted without being individually provided.
FIGS. 3A to 3D show detailed configurations when the single MZI modulator is fabricated on an LN substrate with an optical waveguide (LN waveguide) formed by titanium diffusion. FIGS. 3A and 3B show configurations using the Z-cut substrate; FIGS. 3C and 3D show configurations using the X-cut substrate; FIG. 3B shows a cross-sectional view taken along line IIIB of FIG. 3A; FIG. 3D shows a cross-sectional view taken along line IIID of FIG. 3C. In the Z-cut substrate 14, a high-frequency center electrode 16 is provided in an upper part of the waveguide 15, and a GND electrode 17 is provided around the high-frequency center electrode 16. When a voltage is applied to the high-frequency center electrode 16, as shown in FIG. 3B, in the vicinity of a waveguide core 18, an electric field occurs in a vertical direction. Since this direction is a polarization direction 19 of the Z-cut substrate, a refractive index is changed by Pockels effect, with the result that the phase of the light which propagates through the waveguide is changed. In the X-cut substrate 20, the high-frequency center electrode 16 is provided in an upper part of the middle of the both-arm waveguide 15 of the MZI, and the GND electrode 17 is provided in an upper part around the both-arm waveguide. With such electrode arrangement, as shown in FIG. 3D, in the vicinity of the core, an electric field can be made to occur in a horizontal direction which is the polarization direction 19 of the X-cut substrate, and it is possible to perform phase control on the propagated light as same as with the Z-cut substrate. Note that, in the high-frequency center electrode 16, in order to correspond to the direction in which the light propagates within the waveguide, one end is used as a signal input terminal, and a termination resister 21 is connected to the other end to form a traveling wave electrode structure, thereby enabling extremely high-speed modulation. Moreover, although not shown in this figure, separately from this high-frequency electrode, an electrode of a lumped constant electrode structure may be provided for the adjustment of an operating point. Furthermore, in the subsequent figures, regardless of the Z-cut substrate or the X-cut substrate, as necessary, the single MZI modulator will be simply shown as in FIG. 2D.
The configuration and the operation of a conventional 16QAM modulator will now be described with reference to FIGS. 4A and 4B. FIG. 4A shows the configurations of the 16QAM modulator; FIG. 4B shows the signal constellation of a light signal at points A to C in the modulator of FIG. 4A. The 16QAM modulator includes: a 2:1 optical coupler 22a having one input and two outputs; two QPSK modulators 1a and 1b; two optical phase adjusters (variable optical phase shifters) 12e and 12f and a 2:1 optical coupler 22b having two inputs and one output. The input CW light 6 is branched by the 2:1 optical coupler 22a, and the branched CW lights are respectively QPSK-modulated by the QPSK modulators 1a and 1b to be combined by the 2:1 optical coupler 22b. Since the ratio of the electric field amplitude of a QPSK signal 1 through the QPSK modulator 1a to that of a QPSK signal 2 through the QPSK modulator 1b is 2:1, the relative phase of the QPSK signal 1 and the QPSK signal 2 is appropriately adjusted by the phase adjusters 12e and 12f, and thus it is possible to generate a 16QAM signal as shown in FIG. 4B (NPL 1).
The configuration and the operation of a 2-subcarrier multiplexing QPSK modulator will now be described with reference to FIGS. 5A and 5B. FIG. 5A shows the configuration of a conventional 2-subcarrier multiplexing QPSK modulator; FIG. 5B shows the spectrum and the signal constellation of signals at points A to F in the modulator of FIG. 5A. The 2-subcarrier multiplexing QPSK modulator includes: an interleaving optical filter (ILF) 23 having one input and two outputs; two QPSK modulators 1a and 1b; two optical phase adjusters (variable optical phase shifters) 12e and 12f; and a 3 dB optical coupler 7g having two inputs and one output. The ILF includes a delay line of an optical length difference ΔL inserted between the two 3 dB optical couplers 7a and 7b and has a periodical transmission characteristic in which a free spectral range (FSR) is c/ΔL (here, c is the speed of light) by known interference principles, and it is possible to demultiplex the interleaved input light with frequency interval Δf=FSR/2 and output it. A transmission characteristic TB to the B side and a transmission characteristic Tc to the C side are expressed as formula below.
                                          T            B                    =                                    1              2                        ·                          {                              1                +                                  sin                  (                                      2                    ⁢                    π                    ⁢                                                                                  ⁢                                          f                      FSR                                                        )                                            }                                      ⁢                                  ⁢                              T            C                    =                                    1              2                        ·                          {                              1                -                                  sin                  (                                      2                    ⁢                    π                    ⁢                                                                                  ⁢                                          f                      FSR                                                        )                                            }                                                          Formula        ⁢                                  ⁢        1            
The CW light of two wavelengths in which frequencies are Δf apart is input to this modulator as a subcarrier. Here, when it is assumed that the wavelengths of the CW light are f1 and f2, f1=k·FSR and f2=(k−0.5) FSR where k is an integer. As shown in FIG. 5B, the input subcarrier of the two wavelengths is demultiplexed by the ILF 23, and the demultiplexed waves are individually QPSK-modulated by the respective QPSK modulators 1a and 1b to be combined by the 3 dB optical coupler 7g. In this way, it is possible to generate a subcarrier multiplexed signal (NPL 2).